Ultrasound scanning is finding increased usage in the examination of the peripheral vascular (PV) structure of the human body. Both sector and linear scanning may be used, and each form of scanning may be generated by mechanical or electronic scanners. The PV target area may be illuminated by bursts, or continuous waves of ultrasound energy, depending on the parameters to be measured. In either case, a beam plot of the transmitted wave would show a beam, generally centered at the middle of the transducer, having a region of high energy or high sensitivity at its center. Moving off of the center of this beam, the plot would show regions of lower energy or sensitivity. Such lower sensitivity portions of the transmitted beam might include sidelobes. A similar beam plot could be constructed for received energy.
The vessels to be examined as part of a PV scan are often near the surface of the subject's body. When sector scanning is used for such an examination, it is necessary to have the transducer be some distance from the skin surface of the subject. This is due to the "fan-shaped" nature of the sector scan. If the sector scanner were held against the subject's skin, only a small portion of the beam would intersect the PV region of interest. Holding the sector scanning transducer off the skin affects the beam in two ways. First, the beam becomes wider at the intersection of the region of interest. In addition, the effects of ring down noise caused by the generation of the transmitted pulse are reduced.
If the sector scanning transducer were simply held off the subject's skin, with nothing but air between the transducer and the subject, there would be an inefficient coupling of ultrasonic energy into the region of interest. To prevent this inefficient energy transfer, stand-off devices are commonly used to place the sector scanning transducer at the right position and efficiently couple energy to the region of interest. One such device is described in U.S. Pat. No. 4,579,123, assigned to the assignee of the present application. Stand-off devices may be separate structures, to which a transducer is mechanically or acoustically coupled, or the device may be an integral part of the transducer structure.